Issue an error if transitive visibility differs

[alecthomas]

eg. In this scenario In will be implicitly private due to its use by Private(), but needs to be subsequently exported by its use by Exported().

type In struct {}

//ftl:verb
func Private(ctx context.Context, in In) error {}

//ftl:verb export
func Exported(ctx context.Context, in In) error {}

Same should be true of any visibility propagation.